During the process of creating an AWS Auto Scaling group, you will need to create a Launch Configuration to describe how EC2 instances will be created. When configuring the storage options, new volumes can be added and can be optionally be based off a snapshot. However, there is no option to attach an existing volume.
The reason behind this is a volume can only be attached to a single EC2 instance and an Auto Scaling Group is likely to have multiple instances created from the same Launch Configuration. However, in the scenario where a group is being scaled between 0 and 1 instance, there is no reason why you can't add a volume to the instance when you scale up to 1 instance. I had this scenario with a dev server which was scaled to 0 instances at night and back to 1 instance during working hours.
To get a volume to attach to an EC2 instance when it is created, add the following Powershell to the UserData property of the Launch Configuration. This ensures each time an instance is created, the volume will be available in the same state as the previous day.
If you are using CloudFormation, this would be added to the template as follows:
Tags : AWS